Environmentally harmful species in exhaust gas emitted from an internal combustion engine, such as hydrocarbons (HC), carbon monoxide (CO), particulate matters (PM), and nitric oxides (NOx) are regulated species that need to be removed. In lean combustion engines, e.g. diesel engines, due to their lean combustion nature, PM and NOx are two major emissions. To remove these harmful species, a variety of technologies have being used. Among them, DPF technology is effective in decreasing PM, including both particle mass and numbers, while a number of technologies, including LNT (Lean NOx Trap) and SCR (Selective Catalytic Reduction) are used for reducing NOx emissions.
In a DPF, trapped PM accumulates and increases engine back pressure. To avoid excessively high engine back pressure, the trapped PM in the filter needs to be controlled lower than certain amount. A process for reducing PM is also called a regeneration process. A DPF regeneration can be performed either continuously, during normal operations of the filter, or periodically, after a pre-determined amount of PM has been accumulated. Typically to ensure that a DPF can be reliably regenerated, periodical regenerations are required. And to effectively remove accumulated PM, exhaust temperature needs to be elevated to a certain level, for example, to effectively oxidize PM with oxygen, typically exhaust temperature needs to be controlled above 500° C.
A variety of devices, including electrical heaters, DOCs (Diesel Oxidation Catalysts), and fuel burners, can be used in heating exhaust gas in a DPF regeneration process. And to control exhaust gas temperature to a pre-determined level, the power applied to the devices, e.g. electrical current for electrical heaters, fueling rate for fuel burners, and fuel dosing rate for DOCs, needs to be controlled by an ECU (Engine Control Unit) in response to a few control parameters, such as exhaust gas flow rate, which is normally calculated with engine operating parameters, DOC temperature, and DPF temperature. These control parameters, together with a pressure drop across a DPF, which can be measured with a differential pressure sensor, are also used in estimating PM loading in the DPF, and the PM loading amount value can be further used in triggering DPF regenerations.
In the control parameters, normally the engine operating parameter values are obtained from engine controls. However, the engine operating parameters are not always available, and in some systems, even though the engine operating parameters are available, their applications are limited due to the limits of the system structure. For example, in applications with mechanically controlled engines, e.g. in a vehicle retrofit, ECU and the engine operating parameters are not available since engine fueling is controlled mechanically. In engine systems with multiple exhaust branches, e.g. in a high horse power engine system, even the overall exhaust flow rate can be estimated with the engine fueling rate and engine speed, exhaust flow rate in each branch is not available. In these applications, to control DPF regenerations, either more sensors, such as engine speed sensors and throttle position sensors, are installed in the engine system for obtaining the engine operating parameters, or more assumptions are used in estimation, e.g., assuming exhaust flow is equally distributed in each exhaust branch. Installing new sensors in an engine system changes system structure, causing reliability issues, while more assumptions deteriorate control performance and diagnosis capabilities. Moreover, when sensors are installed in the engine system, different engine types and applications require different sensor types, resulting in high system cost and engineering cost.
When a DOC is used in heating exhaust gas, due to the limit of its light-off temperature, when exhaust gas temperature is low, fuel dosing has to be disabled, since unburnt fuel leaks through the DOC and DPF, creating an emission by itself. As a result, under certain operating conditions, such as when an engine is idling or when a vehicle frequently stops and goes (e.g. a city bus), low exhaust temperature stops DPF regenerations, causing high PM load in the DPF and un-uniform distribution or mal-distribution of PM, which are major causes of thermal runaways damaging the DPF.
In a DPF regeneration process, when exhaust flow rate is low, due to the low thermal energy the exhaust flow carries, a DPF could be locally heated, resulting in un-uniform PM distribution in the DPF. To prevent the PM un-uniform distribution caused by low exhaust flow rate, normally in addition to a temperature threshold, a flow threshold is also applied for disabling heating the DPF. However, as that mentioned above, the flow threshold may delay or interrupt a DPF regeneration when an engine runs into low exhaust flow modes, resulting in high PM accumulation and un-uniform PM distribution, which increase the risk of thermal runaways. Additionally, when exhaust flow rate decreases significantly in a short period of time, e.g. when an engine drops to idle, less heat is carried away by the exhaust flow. In a DPF regeneration, when the exhaust flow rate drops too low, even fuel dosing is disabled, a temperature spike could still be created in the DPF, igniting loaded PM and causing a thermal runaway if there is a high PM accumulation or a PM mal-distribution.
To solve the problems mentioned above, it is then a primary object of the present invention to provide an apparatus to obtain key parameter values in DPF regeneration control without using engine operating parameters, so that the DPF system is able to work when these parameters are not available.
A further object of the present invention is to provide an apparatus to regenerate a DPF with low temperature exhaust gas flow, so that more chances can be obtained for regenerating the DPF.
Another object of the present invention is to provide an apparatus to increase exhaust gas flow rate when an engine operates at low exhaust flow mode, so that DPF regenerations need not to be interrupted at these operating modes.